49 patients with spondylolisthesis isthmica were treated in the years 1991-2006. The age of patients varied between 21 and 65 years, 43.7 year on average. Two stages operative protocol consisted of reduction of the slip by pedicular implant and anterior interbody fusion with the cages was used. The evaluation was based on x-rays height of the lumbar intervertebral foramen and posterior disc height in the 2 distal lumbar vertebrae pre and post operatively. This results showed clearly a decrease of both these values on the level of the slip, demonstrating the lumbar intervertebral stenosis. Te operative procedure decomprssed the lumbar intervertebral foramen and increased both values evaluated Postoperatively in the neighbor vertebrae a height lumbar intervertebral foramen was increased but posterior disc height a unchanged.